In order to determine the rotational speed of an electric motor for the purpose of speed regulation, magnetic or optical transmitters are known that are attached to the shaft of the motor and that are read by magnetic and/or optical sensors on the stator housing. However, such devices tend to have a high cost.
For electric motors that are operated using pulse width modulation (PWM) voltage supply, i.e., in a pulsed fashion, a simpler and more cost-effective method is known to measure the voltage produced by the generator in the non-activated phases, the level of said voltage being proportional to the rotational speed of the motor.
However, the level of the voltage is not a very precise measure of the speed, so this method is not suitable for precise regulation.
German Published Patent Application No. 32 26 614 describes the detection of the ripple content of the current for a circuit arrangement for extending and retracting an automobile antenna in a motorized fashion. If the frequency of the ripple content decreases considerably and finally becomes zero, this event signals that the speed of the motor has approached zero, i.e., the antenna has reached a stop. Here, the speed of the driving motor is not regulated, but rather merely rough information is gathered as to whether the driving motor is rotating or standing still. In addition, this solution is not suitable for motors activated in a pulsed fashion.
A method is known from U.S. Pat. No. 5,664,047 in which the ripple content of the current (current ripple) of a PWM-regulated electric motor is to be used to determine the rotational speed. However, the measurement is made more difficult by the constant change between the activated and non-activated phases of the motor. The shorter the current pulses, which depends on the selected pulse frequency, the target speed, and the load on the motor, the less certain the measuring signal becomes, which can result in erroneous control and thus to fluctuations in the speed.